Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions to metal atoms on a substrate without application of an electrical current. Electroless plating processes and compositions are found in a wide variety of commercial practices and are used for plating a substantial number of metals and alloys onto various substrates. Examples of materials commonly plated through this process can include copper, nickel, gold, cobalt, tin-lead alloys, etc. The substrate surface can be any surface that is either catalytically active itself or can be activated by a catalyst. Possible substrates common in the past include, for example, metal, diamond, and a variety of polymers. Plating processes can be either selective, i.e., only a portion of the substrate surface is catalytically activated to control precisely where metal deposition will occur, or alternatively can be used to coat an entire substrate surface.
Electroless plating has been widely used in the microelectronics industry for deposition of layers on semiconductor wafers. For example, electroless plating has been used in the past to form adhesion, barrier and capping layers on substrates. For the purposes of this disclosure, a barrier layer is defined as a layer formed on at least a portion of a substrate surface which can prevent contact between the materials located on either side of the barrier layer. For example, a barrier layer can prevent oxidation or otherwise render passive the material covered by the barrier layer, or alternatively can prevent the material contained in a layer located on one side of the barrier layer from diffusing into a layer located on the other side of the barrier layer. For instance, in the microelectronics industry, Co(W)P and NiP are examples of two barrier layers used in the past for the prevention of copper ion diffusion into the substrate as well as for copper passivation.
Electroless plating processes known in the past generally include heating a bath solution to a certain deposition temperature, which generally corresponds to at least the minimum deposition temperature (i.e. the minimum temperature where deposition from that bath to that substrate can occur). After heating, the bath solution is pumped into a plating chamber. In the plating chamber, a substrate having an activated surface is present and the electroless plating begins at or near the time the hot solution contacts the substrate.
The plating process itself includes an induction period followed by a steady-state deposition period. The induction period is the time necessary to reach the mixed potential at which the steady-state metal deposition occurs. The deposition is usually designed to occur in a certain pH and temperature range. In a certain range, the deposition rate is proportional to the bath temperature. As such, most electroless plating processes heat the bath to the highest deposition temperature possible to take advantage of the higher deposition rate and increase process throughput. Bath temperature is one of the most important factors affecting the deposition rate of the layer. In addition to the deposition rate, however, bath temperature can also affect the uniformity and composition of the deposit and hence its properties. As such, temperature control of the electroless plating bath continues to be very important in these processes.
In addition to high throughput, uniformity of the deposit formed on the substrate is greatly desired. In the past, the bath solution has been introduced into the plating chamber via a rotating showerhead which has a slit opening or holes. Because the temperature of the bath is high, the induction period is short, and the deposition can start essentially upon contact of the solution with the substrate surface. This spray method can significantly affect the uniformity of the deposit formed on the substrate, however, due to, among other factors, the flow pattern of the bath solution as it is fed into the chamber. In addition, uniformity of the deposit can be affected by distribution of temperature on the wafer surface itself as the deposit is formed.
Prior art methods of electroless plating have proven problematic in many aspects. For example, a suitable showerhead design has proven very difficult for obtaining high quality products. Variation in head opening shapes and sizes, head rotation speed, and flow rate can cause different flow patterns across the substrate surface and affect deposition uniformity. For instance, one region of the substrate can have greater exposure to the high temperature bath solution and subsequently can have more material deposited at those regions. The design of the showerhead has thus been very important in an attempt to obtain even distribution of the solution.
In addition, the high temperature of the bath itself has often been disadvantageous in these processes. For example, loss of water from the bath due to evaporation can cause a change of concentration of the components and consequent change in deposition rate. In order to avoid this, the composition of the bath solution must be closely monitored and water must be frequently replenished. Additionally, reducing agents used in baths will often experience accelerated decomposition at high temperature, thus the lifetime of the bath solution can be shortened due to the high bath temperature. Moreover, in the processes of the past, a large volume of solution is usually held at high temperature and recirculated through the system for each subsequent plating operation. Such systems can demand high energy inputs and create high operating expenses.
Prior art processes also often require a long preparation time prior to deposition. Usually the tank size for the bath solution is 10 gallons or larger for industrial use. Such a tank size requires a long period of time for heating the solution from room temperature to deposition temperature. In addition, after the process is finished and the heater is shut down, the solution will often be circulated for a long period of time until cooled sufficiently to avoid excess bath decomposition.
As such, there is a need for an improved electroless plating process which can provide high quality, uniform deposition on a substrate at a high level of throughput, as well as increase bath lifetime and decrease energy requirements of the system.